1. Field of the Invention
The present invention relates to integrated circuits, and more particularly to input/output buffers of integrated circuits.
2. Description of the Related Art
A differential output buffer on an integrated circuit may drive differential signal lines, e.g., traces on a printed circuit board, which are finite-length transmission lines. To reduce signal reflections on those differential transmission lines and corresponding degradation of signals on those transmission lines, the finite-length transmission lines are coupled to a termination impedance that makes the finite-length transmission lines behave as if they are infinite in length, i.e., the individual transmission lines are terminated by an impedance having a value approximately equal value to the characteristic impedance of a respective transmission line. By integrating termination resistors into the differential output buffer on the integrated circuit, the number of external resistors included on a printed circuit board including the integrated circuit may be reduced.
Although typical termination resistors included in the differential output buffer are designed to be equal to the input impedance of the load to which it is coupled, the actual output impedance may not match the input impedance of the load exactly, due to integrated circuit manufacturing process variations. Accordingly, reflections may degrade the signal and the single-ended, peak-to-peak output voltage level may vary according to the variation in resistor values, thereby reducing the power transferred to the load. Some techniques for impedance matching the load result in mismatches in the output impedances of individual nodes of a differential node, which may degrade the signal and power transfer to the load. In addition, currents flowing through termination resistors may dissipate substantial amounts of power, which decreases battery life in portable applications and increases costs related to packaging and cooling systems.